DNG Converter DNG files are 30% larger than Canon compressed CR3 (C-raw) input files

[tl_t]

Hi, I have a newer Canon camera that uses the .CR3 RAW file format.  You can take your RAW pictures in 2 modes:  full uncompressed CR3, and compressed CR3 (aka, C-raw).  Adobe's official support for CR3 is documented here.  They also support my specific camera, the EOS M6 Mk ii.  

Problem:  When I run the Adobe DNG Converter 12.1 (current version) on a compressed CR3 C-raw file, the output DNG file is about 35% larger than the input file.   In contrast, when I use DNG Converter with a full uncompressed CR3, the output DNG file is about 10% smaller than the input file.

Why is the DNG Converter producing output files that are larger than compressed CR3 input files? 

[JP Hess]

The DNG converter works on an algorithm that doesn't take into consideration whether or not the original raw file is compressed or not compressed. The DNG converter simply converts the image based on that algorithm. The size of the resulting file will be what it will be, and it varies from one raw file type to another. Some raw files are reduced by as much as 50% while others are reduced by much less. And, as you have discovered, some raw files will increase in size based on how they were originally constructed. There is nothing wrong with the converter and nothing you can do about it.

[tl_t]

I wonder if the reason the CR3 C-Raw files come out larger, is that the DNG Converter basically unpacks the RAW bits--including that it DE-compresses them for conversion--and then does NOT RE-compress them since they don't have Canon's proprietary algorithm for the C-RAW compression?  

To put that in simpler terms:  I wonder if DNG converter simply "unzips" the compressed C-Raw file, converts it, and then is unable to "re-zip" the compressed bits?  

None of this is ultimately a make-or-break issue for me.  However, it is slightly inconvenient.  The reason I use the CR3 compressed C-Raw format is to save space, both on my camera card and when I move to my PC.  Using DNG converter basically eliminates the space savings on the PC.  I'll keep using it because I want to convert to DNG standard files, but would be a nice-to-have if DNG converter could somehow convert while maintaining the compression level of the bits. 

[JP Hess]

If saving space is of great concern to you, have you considered using the lossy compression option in the DNG converter? 

The other thing you might consider is adding an external hard drive. They are quite reasonable these days.

[tl_t]

I've never experimented with the DNG lossy compression, didn't realize that existed until now.  I see it's an option in the converter.

If I use that--to minimize impact on image quality--is it better to use the full uncompressed CR3 file format in the camera, so that the DNG lossy compression is operating on the full set of original bits?

Otherwise, the concern would be I'm running DNG lossy compression on top of the CR3 C-raw lossy compression, possibly impacting image quality more than I'd want to.  I'll experiment both ways and see what happens. 

[TheDigitalDog]

Frankly, with the cost of storage space, I see little reason for lossy DNG compression. If your main concern is file size, just leave the CR3’s as is. But there ARE some advantages to DNG over just file size. Case in point:

https://www.cnet.com/news/adobe-offering-new-reasons-to-get-dng-religion/

http://digitaldog.net/files/ThePowerofDNG.pdf

[tl_t]

Yep I agree with you.  Ultimately I want to have my cake and eat it too :-):   I want the benefits of DNG, AND the reduced file size.  But forced to make a choice, I'll stick with DNG and give up the reduced file sizes.

On the question of storage cost, yes and no.  You hear that get repeated a lot, but it depends.  If you're talking QUALITY storage--a fast SSD that is worthy of Lightroom classic and a large RAW photo collection, not to mention one that is durable and well made--then THAT kind of storage really is not cheap.  And for my photos, that's what I use.  I just recently bought a 2 TB SSD most of those are running in the $280 - $350 range, for 2 TB.  

So yeah, I'll probably stick with DNG when push comes to shove.  But quality, fast storage, is actually not all that cheap. 

[JP Hess]

There is no performance benefit to having your images on a SSD. There IS some benefit to having Lightroom (the program itself) and the catalog on SSD, but you would do just as well to have the images on a good quality spinning hard drive with reliable backup. The images are not accessed by Lightroom, all action is performed in the catalog.

[tl_t]

Well kind of.  Yes you CAN run your photos on an HDD or even a NAS, but you will see performance gains by using SSD depending on your applications.  In my case, it's not only Lightroom that's touching the files and SSD.  I have a Canon utility that download images from camera to SSD:  an intensive r/w operation.  Then, and Idrive client that scans the SSD to backup to local NAS, and to cloud (a read operation).  Then in the near future, a Dropbox client that will read the photos on disk and sync to the cloud.  I don't think running on HDD kills your performance or anything, I've run photo apps with the source files on HDD and NAS before, but with applications like these, SSD will give a performance boost. 

One other thing is that Lightroom does access the files:  it stores XMP metadata inside the DNG files themselves if you enable that option in the Preferences (which I do).  

[JP Hess]

If you're using apps that access the original files then I suppose having the files on SSD could provide some benefit. I have chosen not to do that, but to rely on the Lightroom catalog. 10 years of experience doing that hasn't disappointed me so I'm not going to change at this point. Cataalog performs very well on the SSD, with images stored on spinning hard drives.

[tl_t]

Thanks, that makes sense.  Yes I've read here about others as well being able to run catalog ssd, photo files on separate HDD, with good success.   That's also a nice way to keep your storage costs under control, as SSD cost per GB is still *much* higher than traditional HDD.   Have you ever tested the theory of storing the photos on external/network NAS drive?  I tried this at home, it works fine, but even though I have gigabit wired ethernet from my PC to the NAS, it's fairly slow.  In my case, I suspect the issue causing the latency is not the network, but the WD "red" HDD's that only spin at 5400 RPM, plus the older synology CPU.  I've been thinking of upgrading my synology and if I do, may go back to the idea of trying to store photos on the external NAS, if the performance is acceptable.  The cost per GB to store on NAS-based HDD's is *so* much less than trying to buy SSD's for all the photo storage.  

[landyiscandy]

Hi, I had the same problem and I've discovered something super weird. When I convert the files on import with LR the CR3 files will slightly reduce in size. When I convert the CR3 files to DNG with "Adobe DNG Converter" they increase slightly in size. (Not nearly the 30% size difference, they increase maybe 3% in size.) But I don't understand why, since when I "copy as DNG" in LR on import the files slightly reduce in size.

[JP Hess]

Although Adobe touts the benefits of converting to DNG, I rarely do it. When I do, it is more for experimental reasons than anything else. For the most part I leave my images in their original NEF state. I suppose there are arguments for and against either way. There is no right or wrong answer, really. I don't think disk space is really an issue. It's really what ever makes the user feel confident.

If you are one who prefers to write metadata to changes in Lightroom, the benefit of the DNG is that there would not be an XMP file. The metadata would be written directly to the DNG file. Personally, I have always just relied on the Lightroom catalog to retain the metadata so I don't have XMP files anyway and I haven't ever regretted it. That's my personal choice. If I ever need an XMP file for an individual image it's easy to create.

Added later: my apologies for commenting so much about Lightroom. I forgot that I was in the Camera Raw forum.